If you're painting your auto yourself, you might need a little help if you've never done it before, especially when it comes to the primer sealer and apply your first base coat of paint. This video will show you how to do just that. An OEM fender was already prepped by sanding with 500 grit sandpaper on a DA with a soft pad and surfing the edges with a maroon scuff pad.
The first method deals with painting the car part before it's installed onto the vehicle. This allows the painter to paint the part with less masking, which makes a smother and cleaner finish. The problem with this method is that care must be taken when installing the part without damaging the new painted surfaces. Normally, adjacent panels will still require blending to provide an invisible repair.
The second method is to edge the panel, then install it on the vehicle. Then the panel is masked with the blending area and the panel can be painted while blending the adjacent panels. This makes panel alignment easier and any scuffed made while installing the panel can be easily repaired during the refinish process.
Both methods are effective and have pros and cons. However, many auto body shops use one or the other, so it's best to know both ways.